The Twilight Series, part 1: Diapausers in Wilkinson Basin

With the thrill of the lobster molt wearing off, the time has come to crunch some numbers. A few weeks ago--the 22nd of July, 2010 to be more precise--we spent a full day on our deep station in Wilkinson Basin. For six relentless hours, we sampled profile after profile at the same station. What began as a routine and sunny carefree day slowly morphed into grueling repetition, and as twilight descended upon us, so did the madness of tedium. (I won't speak here of what that madness led to.)

The driving motivation behind this long series of profiles was to capture the twilight transition in Wilkinson Basin. Every night, an unfathomable host of plankton emerges from the depths of the world's ocean to feed at the surface under the cloak of darkness. This transition occurs, for the most part, during the twilight hours--hence the name of this sampling series. Our objective is to sort out some of the major players in this massive migration, and to pinpoint as well as we can their preferred depths.

Naturally, we are in the early stages of unraveling this depth-stratified tapestry of plankton, but some curious signals are already appearing. The plot shown below is taken from a night sample, well after the sun has set. On the left is a plot from our laser optical plankton counter. It shows the depth concentration of each size class of plankton--divided by equivalent spherical diameter (ESD). You can see that most of the critters we observed are near the surface, with the concentration tapering off around a depth of 20-40 meters. What stands out is the strong signal at the size range 1000-2000 microns (1-2 mm) at depths below 120 meters. I have labeled this "Diapausing finmarks"-- what I believe to be the copepod Calanus finmarchicus, gathered in large numbers in its deep hibernation. This is something that we hope to confirm with the net tows.

It is interesting to note that, while we think of this species as a cold-water animal at the southern edge of its range here in the Gulf of Maine, it appears to be actively avoiding the coldest depths at this station. The right-hand plot shows a temperature profile, with a classic Maine-intermediate-water signal. I've labeled that. It's a cold layer that forms between two warmer layers in the ocean, due to a combination of processes.

The other curiosity to this temperature plot is the blip of warmer water around 50 meters depth. This I've labeled with a question mark. It appears in nearly all of our profiles, at the same depth, and I've since noticed it in other temperature profiles from the Gulf of Maine. I have yet to find an explanation. Thus, another mystery has bubbled up to the surface.